14: Since the condition is of the form "a product is positive (or zero)", there are two cases. Either both factors are positive (or zero), or both are negative (or zero). The easiest way is to split the situation into two cases according to x.

If x < 0, then the condition sin(πx) ≤ 0 leads to

If x ≥ 0, then the condition sin(πx) ≥ 0 leads to

Since these were two independent possibilities, we take union.

Answer